In next generation multimedia mobile communication systems, which have been actively studied in recent years, there is a demand for a system capable of processing and transmitting a variety of information (e.g., video and radio data) in addition to the early-stage voice service. Also, a 3rd generation wireless communication system is followed by a 4th generation wireless communication which is currently being developed aiming at support of a high-speed data service of 1 gigabits per second (Gbps) in downlink and 500 megabits per second (Mbps) in uplink. Wireless communication systems are designed for the purpose of providing reliable communication to a plurality of users irrespective of users' locations and mobility. However, a wireless channel has an abnormal characteristic such as path loss, noise, fading due to multipath, inter-symbol interference (ISI), the Doppler effect due to mobility of a user equipment, etc. Therefore, various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
Examples of techniques for supporting reliable high-speed data services include Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), etc.
An OFDM system capable of reducing an inter-symbol interference effect with a low complexity is taken into consideration as one of post-3rd generation wireless communication systems. In the OFDM, serially input data symbols are converted into N parallel data symbols, and are then transmitted by being carried in N separate subcarriers. The subcarriers maintain orthogonality in a frequency dimension.
The MIMO technique improves data transmission/reception efficiency by using multiple transmitting antennas and multiple receiving antennas. Examples of the MIMO technique include spatial multiplexing, transmit diversity, beamforming, etc. A MIMO channel matrix depending on the number of receiving antennas and the number of transmitting antennas can be decomposed into a plurality of independent channels. Each of the independent channels is referred to as a layer or a stream. The number of layers is referred to as a rank.
The existing MIMO system is designed based on a structure of multiple codewords (CW). In the structure of multiple codewords, maximum two codewords are transmitted at the same time. Since each codeword has its hybrid automatic repeat request (HARQ) process, new data indicator (NDI), and modulation and coding scheme (MCS), separate control signaling is required.
FIG. 1 is a diagram illustrating a downlink MIMO structure in an existing system. In FIG. 1, two transport blocks (TB) are mapped into two codewords by a transport block to codeword mapping rule. This mapping rule can be configured as illustrated in Table 1 below, in accordance with a swap flag.
TABLE 1Mapping of TB into CWSwap flagCWTB1 −> CW1TB2 −> CW202TB1 −> CW1TB1 −> CW212TB1 −> CW1 (TB2 represents ‘0’ TB.)01TB2 −> CW1 (TB1 represents ‘0’B.)01
As illustrated in Table 1, a transport block may have a size of 0. If a size of a transport block is 0, the corresponding transport block is not mapped into a codeword.
In the existing MIMO system, each codeword can include maximum two layers, and two codewords can be transmitted at the same time. Each transport block can be transmitted through a random codeword by using a swap flag as illustrated in Table 1. Accordingly, acknowledgement/negative acknowledgement (ACK/NACK), HARQ process and NDI are tied into a transport block not a codeword.
FIG. 2 is a diagram illustrating a codeword to layer mapping relation according to the related art. As illustrated in FIG. 2, codewords can be mapped into layers in accordance with the number of ranks and the number of transmitting antennas. After the codewords are mapped into the layers, a MIMO process is performed.
The following Table 2 illustrates an example of layer to cordword mapping.
TABLE 2CW1CW2Rank 1{1}—Rank 2{1}{2}Rank 3{1}{2, 3}Rank 4{1, 2}{3, 4}
In Table 2, numbers in parentheses represent layers.
If eight transmitting antennas are supported by improving the existing system, the number of transport blocks reaches two times of the existing system. Therefore, the existing single user (SU) MIMO structure cannot be extended to support eight transmitting antennas. Also, since transmission of maximum eight layers should be supported, a codeword to layer mapping rule should newly be designed.